style.css

body {
    padding: 10px;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
}

.carousel {
    position: relative;
    width: 398px;
    padding: 10px 40px;
    border: 1px solid #CCC;
    border-radius: 15px;
    background: #eee;
}

.carousel img {
    width: 130px;
    height: 130px;
    /* make it block to remove space around images */
    display: block;
}

.arrow {
    position: absolute;
    top: 60px;
    padding: 0;
    background: #ddd;
    border-radius: 15px;
    border: 1px solid gray;
    font-size: 24px;
    line-height: 24px;
    color: #444;
    display: block;
}

.arrow:focus {
    outline: none;
}

.arrow:hover {
    background: #ccc;
    cursor: pointer;
}

.prev {
    left: 7px;
}

.next {
    right: 7px;
}

.gallery {
    width: 390px;
    overflow: hidden;
}

.gallery ul {
    height: 130px;
    width: 9999px;
    margin: 0;
    padding: 0;
    list-style: none;
    transition: margin-left 250ms;
    /* remove white-space between inline-block'ed li */
    /* http://davidwalsh.name/remove-whitespace-inline-block */
    font-size: 0;
}

.gallery li {
    display: inline-block;
}
.menu ul {
    margin: 0;
    list-style: none;
    padding-left: 20px;
    display: none;
}

.menu .title {
    font-size: 18px;
    cursor: pointer;
}

.menu .title::before {
    content: '▶ ';
    font-size: 80%;
    color: green;
}

.menu.open .title::before {
    content: '▼ ';
}

.menu.open ul {
    display: block;
}